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Advisory Guide to the Interpretation of VAERS Data

When evaluating data from VAERS, it is important to note that for any reported event, no cause and effect relationship has been established. VAERS is interested in all potential associations between vaccines and adverse events. Therefore, VAERS collects data on any adverse event following vaccination, be it coincidental or truly caused by a vaccine. The report of an adverse event to VAERS is not documentation that a vaccine caused the event.

Over ten million vaccinations per year are given to children less than one year old, usually between 2 months and 6 months of age. At this age, infants are at greatest risk for certain medical events, including high fevers, seizures, and sudden infant death syndrome (SIDS). Some infants will by coincidence experience such an event shortly after a vaccination.

These coincidences make it difficult to know whether a particular adverse event resulted from a concurrent condition or from a vaccination. Therefore, doctors and other vaccine providers are encouraged to report all adverse events following vaccinations, whether or not they believe that the vaccination was the cause.

VAERS data have a number of limitations you should remember:

VAERS data are derived from a passive surveillance system and represent unverified reports of health events, both minor and serious, that occur after vaccination.

Such data are subject to limitations of under-reporting, simultaneous administration of multiple vaccine antigens (making it difficult to know to which of the vaccines, if any, the event might be attributed), reporting bias, and lack of incidence rates in unvaccinated comparison groups.

While some events reported to VAERS are truly caused by vaccines, others may be related to an underlying disease or condition, to drugs being taken concurrently, or may occur by chance shortly after a vaccine was administered.

VAERS occasionally receives case reports from US manufacturers that were reported to their foreign subsidiaries. Under FDA regulations, if a manufacturer is notified of a foreign case report that describes an event that is both serious and unexpected (in other words, it does not appear in the product labeling), they are required to submit it to VAERS. It is important to realize that these case reports are of variable data quality and completeness, due to the many differences in country reporting practices and surveillance system quality.

In some media reports and on some web sites on the Internet, VAERS reports are presented as verified cases of vaccine deaths and injuries. Statements such as these misrepresent the nature of the VAERS surveillance system.

The coding of VAERS reports also requires careful interpretation:

All narrative text taken from VAERS reports are coded and entered into the VAERS database utilizing the FDA's Coding Symbols for a Thesaurus of Adverse Reaction Terms (COSTART) which are key words representing the medical condition(s) described in the case report.

The CDC and FDA take into account the complex factors mentioned above, and others, when monitoring vaccine safety and analyzing VAERS reports.

The purpose of VAERS is to detect possible signals of adverse events associated with vaccines. Additional scientific investigations are almost always required to properly validate signals from VAERS and establish a cause and effect relationship between a vaccine and an adverse event.
For example, potential concerns raised by VAERS are investigated through a CDC project called the
Vaccine Safety Datalink (VSD).
VSD is a large-linked database and includes information on more than six million people, allowing for planned vaccine safety studies as well as timely investigations of hypotheses.
Source: http://vaers.hhs.gov/info.htm accessed on April 25, 2006.

This page last reviewed: Tuesday, January 05, 2016

This information is provided as technical reference material. Please contact us at cwus@cdc.gov to request a simple text version of this document.